Obviously, modifying the software in-house counts as a derivative work, and I'm okay with that - just not with the idea that customers would then onsell or give away the modified product to other potential customers.

Just selling source code and not granting any license at all would probably accomplish that in practice if not strictly by law. In-house modifications are arguably fair use, and even if not it's basically unenforceable law anyway.

Personally I'd want to go one step further. I'd like to allow (and encourage) distribution of modifications (for free or for a charge), to people who already purchased the original.

I was thinking recently that I'd like something similar to this, though I wouldn't call it a "libre" license. The idea is that anyone who purchased the software would get full source code and a license to make any changes without restriction, and they could distribute the software, with or without changes, to anyone who purchased a license. Furthermore, they could resell licenses. So if the base license cost $10, they could make changes and sell the modified software for $15, as long as they turned over $10 to me for each license they sold. Incidentally, it seems to me that such a license is sufficiently "free", and could actually be more useful than traditional free software, because it allows people who make modifications the ability to make money.